Abstract

Industry generated hard data since 1970 has revealed the presence of a compound batholithic arc system oriented northwest-southeast in the western portion of the South China Sea. The late Cretaceous Natuna-Khorat Batholithic Arc and the Paus Ridge remnant of the associated trench are continuous with the Lupar Subduction Zone of western Sarawak (northern Borneo). The West Natuna Sub-basin, which includes portions of Indonesian exploration Blocks "A" and "B", apparently originated in early Oligocene. This basin is the south end of an inter-batholithic basin (Gulf of Thailand) situated between an early Mesozoic batholithic arc and the late Mesozoic Natuna-Khorat Batholithic Arc. As such, the initial basin fill was locally derived from granitic terrane.

The large scala subducation system responsible for Mesozoic accretion along the Lupar Subduction Zone ceased at the end of Eocene time without notable collision or suturing. Thus, the outer (Natuna) batholithic arc elements were not strongly mobilized vertically, and compensatory downwelling for the Gulf of Thailand Basin (including West Natuna) was inadequate for thick marine sediment development. The Oligocene and Miocene sediment fill is characterized by a brackish freshwater influence and low energy clastics.

The late Miocene tectonic style resulted in large scale vertical uplifts and folds related to basement block faulting with a probable wrench component as well. The wrench movement is believed to have originated from major offset between the Khorat and Natuna portions of the late Mesozoic batholithic arc system. Exploratory success in West Block "B" has been confined to structures which exhibit an older (Oligocene) growth history.